What size wire to use from power BOX to electric central herater

Does this heater come with installation instructions? Are we supposed to guess at the amperage required by the heating unit?

What size wire to use from power BOX to electric central herater
Please call an electrician. He'll know exactly what to do.

That size will be determined by two factors. One is the circuit requirement which is given by the manufacturer, and it will be on the label affixed to the equipment. The other factor is the distance between the load center and the equipment. And yes, a professional will be able to tell you what is required.